Msc: Sustainable Engineering : Chemical and Process Engineering
Biomethane to Grid
The upgrading of biogas from anaerobic digestion (AD) is a well-established process, typically resulting in biomethane that is often blended with natural gas within the grid. As a direct replacement for fossil-derived natural gas, biomethane is the only operational technology currently driving decarbonization within the gas grid.
However, for a biomethane plant to connect to the gas grid, it must undergo a detailed analysis study conducted by Gas Distribution Networks (GDN). This evaluations aims to determine allowable flow rates, injection locations, and ensure that the gas meets the specifications set forth by the GDN, which may requires the addition of propane and odorants.
Connecting to Grid
The current process of connecting to a grid via a GDN in Scotland which according to Entry Agreement from SGN (Scottish Gas Networks) for the injection of gas into the Local Distribution Zone (LDZ) Entry point with an inlet pressure of 5.5 bar.
Connecting to Grid process diagram
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1. Biogas exits the Anaerobic Digestion (AD) unit and then treated in the Biogas Upgrading Unit (BUU). The BUU removes Carbon Dioxide and other contaminant using high-pressure water scrubbing
2. The upgraded biogas exits the BUU and fed through the Grid Entry Unit (GEU). The GEU enriches, odorizes, meters, and analyzes the gas, ensuring the produced biomethane meets grid specifications. The GEU and BUU are in the same location
3. The resulting biomethane is then fed through pipeline (5.5 bar) that transports the gas from the AD site to the agreed connection point with the GDN.
4. Gas injection into the grid occurs at the connection point established in the agreement with the GDN Network.
Gas Composition
As a baseline for evaluation, the project results are compared with the current gas grid composition according to the report by government publications (Environment Agency (2016). Material Comparators for Fuels - Natural Gas). The table below describes the current gas grid composition and compares it with Biomethane. Biomethane (product) represents the biomethane from the exit of the scrubbing tower, while Biomethane (to grid) is the flow ready to be injected into the grid after passing through the Gas Entry Unit for the addition of propane and odorant
Comparison between Natural Gas Grid Composition and Project Results
The Calorific Value must fall within the range of 36.9 to 42.3 MJ/m3. This range ensures that the end-user customer is charged a comparable value to natural gas. If the calorific value is lower, customers would pay the same amount for less energy. Therefore, maintaining this specified range is crucial for fairness in pricing and customer satisfaction (Department for Security, Energy & Net Zero [DENZ], 2024). Additionally, the results obtained must meet the base requirements for grid injection, including caloric value, odorization, and temperature, ensuring compliance with the requirement in the gas distribution process.
The government offers financial incentives for renewable gas production through anaerobic digestion (AD) plants, which produce biomethane injected into the grid under the Green Gas Support Scheme (GGSS) policy. This scheme aims to foster a sustainable, efficient, and profitable market for biomethane, providing a clean alternative to fossil fuels
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According to the Office of Gas and Electricity Markets (Ofgem) (2022)-GGSS Guidance The GGSS offers a guaranteed tariff per unit of biomethane production, providing a fixed rate for each kilowatt-hour (kWh) of biomethane injected into the grid . This scheme operates with tiered tariffs, where larger biomethane production volumes receive lower rates. However, there are caps on these tiers to prevent over-compensation for excessively large-scale production. In our project, the production of approximately 4,082 MWh of biomethane qualifies for the highest tier (Tier 1), entitling us to a tariff of 6.33 pence per kilowatt-hour (p/kWh) and potentially yielding a revenue of £258,411. Moreover, if biomethane injection exceeds 250,000 MWh, the plant will no longer receive payments from the scheme.
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GGSS Tariff Table Q1 (2024-2025)
The scheme offers long-term support with tariff payments for 15 years, providing stability for biomethane producers. The application window remains open until March 31, 2028 According to a government response report in a mid scheme review of GGSS.